Controller circuit

ABSTRACT

Temperature controllers including a sensing device, such as a thermocouple which is temperature compensated by means of a negative temperature coefficient resistance, and which generates a signal in response to the temperature of the thermocouple junction; an output circuit for manifesting a first output when a temperature limit is not exceeded and a second output when the limit is exceeded; a circuit which senses power interruption and forces the output circuit to manifest the first output for a limited time after the power is restored; and an alternative output circuit which provides a proportional controlling signal in response to the sensed temperature.

0 United States Patent [1 1 Kohn Dec. 4, 1973 [54] CONTROLLER CIRCUIT3,643,108 2/1972 Pilkington 307/310 X [75] Inventor: Mitchell I. Kohn,Chicago, Ill.

Primary ExaminerJohn Zazworsky i 1 g f l Corporafions Att0meylrvin C.Alter et a1.

Whee ing, 11

[22] Filed: May 1, 1972 [57] ABSTRACT [21] Appl. No.: 249,287Temperature controllers including a sensing device,

such as a thermocouple which is temperature compensated by means of anegative temperature coefficient [52] US. Cl. 307/310, 2211947523,231:.97/729996, resistance, and which generates a Sigma in response toInt Cl 03k 5/20 fiolv l loo the temperature of the thermocouplejunction; an out- [58] Field u 367/310 put circuit for manifesting afirst output when a tem- 499 perature limit is not exceeded and a secondoutput when the limit is exceeded; a circuit which senses powerinterruption and forces the output circuit to [56] References Citedmanifest the first output for a limited time after the UNITED STATESPATENTS power is restored; and an alternative output circuit 3,632,985l/1972 Bare et al. 219/499 which provides a proportional controllingsignal in re- Funfstuck n... X S onse to the sensed tem erature3,523,182 8/1970 Phillips et al. 219/501 p p 3,604,957 9/1971 Satula307/310 11 Claims, 3 Drawing Figures I06 10a stem PATENTED DEC 41975SHEET 2 BF 2 i/IZ 2/8 g2 1 CONTROLLER CIRCUIT BACKGROUND OF THEINVENTION This invention relates to a controller circuit and moreparticularly to a controller circuit adapted to produce an indicatingsignal in response to detection,at a remote location, of a parameterexceeding a predetermined value. The invention also relates toproportioning controller circuits.

Electronic controller apparatus are commonly used as a part of systemsfor controlling temperature, either the temperature within a closedsystem or at one .or more steps of a continuous process which istemperature sensitive. Typical uses of such controllers are inconnection with the operation of heating elements in a furnace or inconnection with the control of refrigeration equipment.

Other uses of such controllers include detection of when a limitingcondition such as a maximum temperature limit is exceeded. When so used,it is frequently necessary to shut down the equipment being operateduntil a malfunction is corrected.

Control circuits heretofore known have been handi capped in theirinability to deal with abnormal circumstances such as interruption ofpower to the machinery being controlled. For example, if the limitresponsive circuit detects a limit condition and manifests that by theoperation of a relay, interruption of power and a subsequentreapplication of power may cause the relay falselyto remain unenergizedeven though the limit condition may remain exceeded. In other cases,interruption and restoration of power may cause the relay falsely toindicate a limit condition when in fact none has occurred.

Previously known control circuits have also been limited with respect tothe sensitivity of control which can be applied to machinery beingcontrolled, in that relatively complicated circuits have been necessaryfor effecting proportional control to turn on and off a control signalin response to the value of a sensed parameter. If a relatively largetemperature range is provided between the temperature at which thecontrol signal is energized, and the temperature at which the controlsignal is de-energized, i. e., a relatively great hysteresis isprovided, the temperature being controlled is free to assume any valuewithin that range. If the hysteresis is made smaller, the sensitivity isincreased, but undesirable oscillation may occur if the sensitivity isincreased by more than a predetermined amount.

Previous circuits for temperature control have also been limited in theaccuracy with which they can respond to changes'in the temperature whichis to be regulat'ed, especially when thermocouples are employed. As thevoltage produced by a thermocouple is a function of the difference intemperature between two junctions, failure to compensate for differencesin temperature at one of the junctions results in inaccuracies. Effortsin the past to eliminate variable temperatures at the reference junctionas a source of error have resulted in rather cumbersome assemblies inwhich the temperature at the reference junction was made fixed. Onemethod commonly used has been to place the reference junction in a bathof ice and water. It is desirable to provide a temperature sensor inwhich variations in the temperature of the reference junction isautomatically compensated form that the output voltage is solelydependent upon the temperature of the sensing junction without actuallyholding the reference junction to a fixed temperature.

Accordingly, it is a principal object of the present invention toprovide a controller having means for causing a limit control to assumea known condition immediately upon resumption of power following anunexpected interruption inpower, and thereafter to change to its othercondition only if the limit remains exceeded following decay of alltransients resulting from reapplication of power to the controlapparatus.

It is a further object of the present invention to provide a controllerhaving a mechanism which is normally disabled when there is a normalapplication of power to the controller, but enabled immediately when thepower has been resumed after an interruption, and disabled again a shortinterval later.

Another object of the present invention is to provide a controller forproducing a control signal in response to the value of a sensedparameter in which the amount of hysteresis in the operation of thecontrol circuit is independent of the value of the parameter.

A further object of the present invention is to provide a proportioningcontroller in which the duty cycle of the proportional controllingsignal is modified in accordance with-the level of the sensed parameter.

Another object of the present invention is to provide a controllerhaving means for producing a signal proportional to the temperature at aremote location, independently of the value of a reference temperature.

A further object of the present invention is to provide means forsensing the temperature at a remote location, and for automaticallycompensating for the temperature at another location so that the valueof the output signal produced in response to the temperature at theremote location is independent of changes in the temperature at theother location.

Another object of the present invention is to provide a controllerhaving apparatus for sensing the temperature at a remote location andmanifesting a first output signal if the remote temperature is greaterthan a first predetermined value and for manifesting a different signalwhen the temperature at a remote location is less than a secondpredetermined value, and means for restoring one of said first or secondsignals following a time interval subsequent to resumption of powerfollowing an interruption.

An additional object of the present invention is to provide a controllerfulfilling the above-named objects which is further characterized byversatility of application, economy of construction, stability ofoperation and simplicity of design.

These and other objects of the present invention will become manifestupon an examination of the accompanying drawings and the followingdescription.

SUMMARY OF THE INVENTION In accordance with one aspect of the presentinvention there is provided, in a control circuit of the type described,circuit means for manifesting an output signal in response to theexceeding of a limit condition, means for detecting a powerinterruption, said detecting means being .connected to said circuitmeans for furnishing a signal thereto following the resumption of powerto cause said circuit means to manifest a signal indicative of acondition in which said limit condition has not been exceeded, and meansto disable said detection means following a predetermined interval afterthe resumption of power for enabling said circuit means to manifest anoutput signal indicative of a limit condition if the limit conditionremains exceeded after said predetermined interval.

In accordance with another aspect of the present invention there isprovided circuit means for alternately manifesting first and secondoutput signals, the duty cycle of said output signals being variable inresponse to the difference between the amplitude of an input signal, anda reference signal.

In accordance with a further feature of the present invention there isprovided a thermocouple adapted to be placed in a remote location wherethe temperature is to be measured, a resistance bridge connected withsaid thermocouple and operative to produce an output voltage in responseto the voltage generated by said thermocouple, a negative temperaturecoefficient component located in thermal proximity to the connectionbetween said thermocouple and said bridge, and means connecting saidcomponent with said bridge, said component automatically compensatingfor the temperature of the junction of said thermocouple with saidbridge to produce an output signal which is independent of thetemperature of said junction.

BRIEF DESCRIPTION OF THE DRAWINGS Reference will now be made to theaccompanying drawings in which:

FIG. 1 is an illustration in schematic form of a portion of a controllercircuit embodying the present invention;

FIG. 2 is an illustration in schematic form of an additional portion ofa first embodiment of the present invention, which cooperates with theapparatus of FIG. 1; and

FIG. 3 is an illustration in schematic form of an additional portion ofa second embodiment of the present invention, which cooperates with theapparatus of FIG. 1.

DESCRIPTION OF THE INVENTION Referring now to FIG. 1 there isillustrated a temperature measuring bridge having a thermocouple 12connected by means of a connector 14 with the remainder of the circuit.The connector 14 permits the thermocouple 12 to be removed and replacedwith another element if necessary. Thermocouple 12 is suitably disposedto sense the desired temperature. The bridge 10 includes a firstresistor 16 connected to a line 18 which is in turn connected to asource of negative potential. The series combination of resistor 16 andresistors 20, 22, 24 and 25 connect line 18 to a common ground circuitpoint. A negative temperature coefficient thermistor 26 is connected inparallel with the resistor 24, and the thermistor is physicallypositioned close to, and in thermal contact with, the connector 14 sothat it assumes the same temperature as that of the junction between thethermocouple leads and the conductors of the connector 14 and any otherelectrical junction of dissimilar metallic materials associated withconnector l4.

The resistor 22 has a rheostat 27 connected in paralv lel therewith, bywhich the range or span of temperatures to be measured may be set. Apotentiometer 28 is also connected in parallel with the resistor 22, andits tap is connected by way of the line 30 through the connector 14 toone lead of the thermocouple l2. Potentiometer 28 provides a circuitadjustment by which the user may adjust the controller set point, i. e.,either the limit temperature to which the controller is to respond orthe value at which it is desired to maintain the sensed temperature. Theother lead of the thermocouple 12 is connected through the connector 14and by means of a resistor 32 to the noninverting input 34 of anoperational amplifier 36. A capacitor 38 is connected from the input 34to ground, to act as a low pass filter, and a pair of reverse-poleddiodes 40 and 42 are connected in parallel with the capacitor 38, tolimit the amplitude of signals applied to the input terminal 34.

A resistor 44 is connected to the junction of the resistors l6 and 20and leads from that junction through a resistor 46 and a resistor 48 toground. A potentiometer 50 is connected in parallel with the resistor 46and the tap of the potentiometer 50 is connected through a resistor 52to the inverting input 54 of the operational amplifier 36. A capacitor56 connects the input 54 to ground to bypass high frequency components.

The operational amplifier 36 is powered by positive and negativevoltages derived from a power transformer 58. The primary winding oftransformer 58 may include a plurality of taps to permit operation ofthe controller with a plurality of different line voltages. Thetransformer also includes an electrostatic shield for connection toearth ground. The secondary winding of the transformer 58 is connectedto two half wave rectifier circuits incorporating diodes 60 and 62 andcapacitors 64 and 66, arranged as shown in FIG. 1. The voltage appearingacross the capacitors 64 and 66 is dropped by resistors 68 and 70 andstabilized by a pair of zener diodes 72 and 74. The anode of the zenerdiode 72, where the negative voltage appears, is connected to thenegative power terminal 76 of the operational amplifier 36 while thecathode of the zener diode 74, where the positive voltage appears, isconnected to the positive power terminal 78 of the operational amplifier36. Capacitors 73 and 75 connect terminals 76 and 78, respectively, ofoperational amplifier 36 to ground, thus bypassing any a.c. signalswhich might otherwise be present at these terminals to ground,neutralizing lead inductances, and improving the stability of theamplifier.

A resistor is connected from the terminal 78 to the junction of thethermocouple l2 and the resistor 32. Resistor 80 is of relatively highimpedance relative to the impedances of bridge circuit 10 and operatesto apply a relatively high voltage to noninverting input 34 of theamplifier 36 should the leads to thermocouple 12 break, thus causing thecontroller output to indicate that the sensed temperature has gonebeyond the set point value, either tripping an alarm signal or shuttingdown the controlled process depending on the use made of the controlleroutput. A capacitor 82 is connected from the output terminal 84 of theamplifier 36 to its inverting input terminal 54 to suppress highfrequency signals.

The output terminal 84 is connected to a terminal 112, and through aresistor 92, a potentiometer 94 and a resistor 96 to ground. The tap ofthe potentiometer 94 is connected through a resistor 98 to the invertinginput 54 of the amplifier 36, to provide negative feedback to establishthe gain of the amplifier. The output is also connected to a circuitincluding series connected resistors 100 and 102 to ground. A meter 104is connected in series with a thermistor 106 across the resistor 102 andis arranged to indicate the voltage present at the output terminal 84.The appropriate voltage range is obtained by selecting the appropriatevalues for the resistors 100 and 102 and thermistor 106, as is wellknown by those skilled in the art. Thermistor 106 compensates forvariations in the resistance of the coil in meter 104 with temperature.The gain of the amplifier 36 is adjustable by means of the potentiometer94, to calibrate the meter 104 with respect to a scale indicating thenumber of degrees that the temperature at the location of thethermocouple 12 differs from the temperature set by the potentiometer28.

The amplifier 36 functions as a differential amplifier that amplifiesthe difference in the voltage, if any, between a first potentialsupplied by the potentiometer 50, and a second potential formed by theaddition of the potential selected by the tap of potentiometer 28 to thepotential produced by the thermocouple 12. When the first and secondpotentials are equal, there is no difference between the inputs 34 and54 and the output of the amplifier 36 vanishes.

In calibrating the apparatus of FIG. 1, the potentiometer 28 is adjustedso that the dial associated with the potentiometer 28 reads the existingtemperature of the thermocouple 12. For the purpose of calibration, thetemperature of the thermocouple 12 may be stabilized at some knowntemperature by means not shown or a suitable equivalent potential sourcemay be interposed therefor. Then, the potentiometer 50 is adjusted sothat the output of the amplifier 36 at the terminal 84 vanishes, asdetermined by the meter 104. The setting of the potentiometer 28 is thenchanged by a few degrees, and the gain of the amplifier is adjusted, bymeans of the potentiometer 94, until the deviation meter 104 indicatesthe number of degrees as the difference between the dial on thepotentiometer 28 and the fixed temperature of the thermocouple. Thecircuit is thus brought into balance. The meter 104 will show thedeviation, in degrees, between the temperature of the thermocouple 12and the setting of the potentiometer 28. The output of the amplifier 36is an error signal which is employed to energize heating or coolingsystems so that the temperature at the thermocouple 12 is brought intoconformity with the setting of the potentiometer 28. As will beapparent, other schemes maybe used to calibrate the circuit of FIG. 1.

The voltage on the line 18 is obtained by a circuit including a resistor106 in series with a rheostat 108 and a zener diode 110, all connectedacross the zener diode 72. The line 18 is connected to the junction ofthe rheostat 108 and the zener diode 110 which may advantageously be atemperature compensated zener diode. The rheostat 108 is employed toadjust the current flowing through the zener diode 110 to optimize thecompensation of zener diode 110 and the regulation on the line 18. Itwill be appreciated that the voltage on the line 18 is very stablebecause of its being obtained by two zener diodes connected in cascade.

Referring now to FIG. 2, the terminal 112 is connected through aresistor 114 to the terminal 116 of a single pole double throw switch118. A jumper system may be used in place of switch 118. The switch 118connects the signal from the terminal 116 to one or the other inputs ofan operational amplifier 120. The inverting input 122 of the amplifier120 is connected to one position of the switch 118 and the noninvertinginput 124 is connected to the other position of the switch 118. Acapacitor 126 is connected between the terminal 116 and ground toprovide a bypass, and reverse poled diodes 128 and 130 are connected inparallel with the capacitor 126 to limit themaximum signal which can beapplied to the amplifier 120. Positive voltage is applied to thepositive power terminal 130 by means of a lead 132 connected with theterminal 78 of the amplifier 36 (FIG. 1), via terminal 79.

Similarly, the negative power terminal 134 is connected to the negativepower terminal 76 of the amplifier 36, via terminal 77, by means of aline 136. Capacitors and 142 connect terminals 130 and 134,respectively, of operational amplifier 120 to ground and performfunctions similar to those of capacitors 73 and 75 in FIG. 1.

A resistor 144 is connected from the inverting input 122 to ground, anda resistor 146 is connected from the noninverting input 124 to ground. Afurther resistor 148 is connected from the noninverting input 124 to alead 150, which is connected either to a source of positive potentialvia a line 152, or to a source of negative potential via a line 154, asrequired, and the value of the resistor 148 is selected to change theoutput of the amplifier 120, to the first condition, as defined below,when a zero level signal is applied to terminal 112. The output of theamplifier 120 is provided at an output terminal 156. A voltage dividerincluding resistors 158 and is connected from the terminal 156 toground, and a resistor 162 is connected from the junction of theresistors 158 and 160 to the noninverting input 124, in order to providepositive feedback. The provision of the feedback resistor 162 makes theamplifier 120 stable in one of two conditions depending upon the valueof the input signal supplied from the terminal 112. In the first oroperated condition, the voltage at output terminal 156 is approximatelythat of the negative supply at power terminal 134, while in the secondor unoperated condition, the voltage at output terminal 156 isapproximately that of the positive supply at power terminal 130. Theswitch 118 determines whether the amplifier 120 is operated fortemperatures above a given limit, or for temperatures below a givenlimit. Considering the case where the switch 1 18 connects the terminal112 to the inverting input 122, the amplifier 120 is normally in thesecond condition for temperatures below a given limit. As thetemperature sensed by the thermocouple l2 rises, the voltage level atthe terminal 112 rises, and the amplifier 120 assumes the firstcondition, the signal at the output terminal 156 then falling sharply.Positive feedback through the resistor 162 functions to maintain theamplifier 120 in its first condition, even if the temperature shouldfall slightly thereafter. The amplifier 120 is returned to its secondcondition only when the voltage level present at the terminal 112 fallsto a value below the threshold value required for operating theamplifier 120. The positive feedback accordingly establishes somehysteresis in the operation of the amplifier 120 which prevents it fromoscillating in response to slight changes in temperature. If a low limitis desired, the switch 118 is operated to its other condition, so that anegative signal is required to operate the amplifier 120. A lessnegative signal is then required to restore the amplifier 120 to itssecond condition.

The output terminal 156 is connected through a resistor 164 to the gateterminal of a silicon controlled rectifier (SCR) 166. The cathode of theSCR 166 is connected to ground and its gate and cathode are connectedtogether by a resistor 168. When amplifier 120 is in its secondcondition (positive potential at terminal 156), the SCR 166 isconductive, and current flows from a source of a.c. potential connectedfrom a terminal 170 through a connector 172, a line 174, the coil of arelay 176, and back through the connector 172 to the anode of the SCR166. When the amplifier 120 is in its first condition (negativepotential at terminal 156), the SCR 166 is cut off at the end ofapositive half cycle of the ac. voltage. A capacitor 177 is connectedacross the coil of the relay 176 to prevent relay chatter and insurethatthe relay coil remains energized during the on-half cycles of power flowthat SCR 166 is nonconductive.

The relay 176 has a common terminal 178 which is connected through theconnector 172 to a line 180. A normally closed circuit extends from thecommon terminal 178 via line 182 through the connector 172 to a line184. A normally open circuit, closed when the coil of the relay 176 isenergized, includes a line 186 which connects through the connector 174with a line 188. The line 180 is connected to a source of power througha fuse 190, and the lines 184 and 188 are connected to furnish power toother apparatus used with the control circuit of the present invention,such as heating or cooling apparatus, indicator apparatus, or the like.

In operation the circuit of FIG. 2 functions to change the state of theamplifier 120 from its second condition to its first condition when alimit has been exceeded, the sign of the limit condition being set bythe switch 118. If a power interruption should occur, reapplication ofpower after a short interval can place the amplifier 120 in either ofits two stable conditions, primarily as a matteer of chance, becausethere is a generally random order in the sequence of which various partsof the control circuit are brought up to operating voltage. It isdesirable to provide a means for insuring that the second condition isassumed by the amplifier 120, follow ing resumption of power, in orderto prevent the power resumption from having the effect of signaling alimit condition when none exists.

The amplifier 120 has an additional circuit for applying positivefeedback when the amplifier 120 is in its first condition. Thiscomprises a series circuit connected from the terminal 156 to ground,including a diode 191, a resistor 192, another resistor 193, and acapacitor 194. A resistor 195 connects the junction of the resistor 193and the capacitor 194 to the noninverting input 124. When the amplifier120 is in its first condition, the capacitor 194 is charged through theresistors 192 and 193, and its voltage is applied to the input 124 viathe resistor 195. The values of the capacitor 194 and the resistors 192and 193 determine the speed at which this positive feedback is applied,and it is designed to be slow enough in operation so that very shortexcursions of the potential at the terminal 112 do not cause theamplifier 120 to latch in its first condition.

A normally open switch 196 is connected from ground to the junction ofthe resistors 192 and 193 to disable the positive feedback provided bythis circuit, and reset the amplifier 120 to its second condition,provided the potential at the terminal 112 no longer exceeds the limitvalue.

The circuit for preventing random operation of the amplifier 120includes a full wave rectifier comprising diodes 197 and 198 (FIG. 1)connected between the secondary of the transformer 58 and a capacitor200, which is connected from the junction of the diodes 197 and 198 toground. The voltage across the capacitor 200 is made available, viaterminal 201, to the circuit of FIG. 2. The terminal 201 is connectedthrough resistors 202 and 203 to ground, and the junction of theresistors 202 and 203 is applied to the gate of a field effecttransistor (FET) 204. A capacitor 206 is connected between the sourceand drain terminals of the FET 204, and the drain of the FET 204 isconnected through a resistor 208 to the line 154 which is connected witha source of negative voltage. The drain of the FET 204 is also connectedthrough a resistor 210 to the gate of an FET 212. The drain of the FET212 is connected directly to the noninverting input 124 of theoperational amplifier 120. A diode 214 connects the source of the FET212 to ground, and its source is also connected to a source of positivepotential at terminal 79 through a resistor 216.

The FETs 204 and 212 are preferably N-channel FETs of the type that areconductive when no potential is applied to their gate terminals. Anegative signal applied to the gate results in the FET being cut off.

In operation, the FETs 204 and 212 are normally cut off by means of thevoltage developed across the capacitor 200 and the voltage across thecapacitor 206, respectively. When the power is interrupted, however, thecapacitor 200 is discharged through the resistors 202 and 203,permitting the FET 204 to become conductive. The capacitor 206 isthereupon discharged through the FET 204, raising the potential of thegate of the FET 212 and permitting the FET 212 also to becomeconductive. Accordingly, the terminal 79 is connected to thenoninverting input 124 of the operational amplifier through the FET 212and the resistor 216. Diode 214 limits the potential at the source ofFET 212 to the forward conductance voltage of the diode. Of course,since the power has been interrupted, the increased conduction of theFET 212 does not increase the signal at the terminal 124, since thepower applied to the terminal 79 is also interrupted. However upon theresumption of the input power, the F ET 212 permits a positive voltagefrom terminal 79 to be applied through the resistor 216 and the clamp ofdiode 214 to the noninverting input 124 of the amplifier 120, placingthe operational amplifier 120 in its second condition. A short timeafter the power connection has been re-established, the capacitor 200 ischarged, cutting off the FET 204, and thereafter the capacitor 206 ischarged through the resistor 208, cutting off the FET 212. The intervalduring which the FET 212 is conductive following resumption of power iscontrollable partly by selecting the value of capacitance of thecapacitor 206 and the value of resistance of the resistor 208. Thesecomponents are selected to give a time interval of approximatelyone-half second to 4 seconds before the FET becomes nonconductive. Thisis sufficient time to permit the voltage levels throughout the controlcircuit to achieve their steady state values, after which the conditionof the operational amplifier 120 is not subject to random variations.The FET 212 then becomes nonconductive again so that the operationalamplifier 120 is free to operate in the normal manner, and will beoperated whenever the limit condition is exceeded. The short time periodof one-half to 4 seconds in which the output of the operationalamplifier 120 may falsely indicate that no limit condition has occurreddoes not produce any adverse effects, partly because of the shortness ofthe interval, and also because the power interruption typically bringsabout a reduction in the value of the parameter whose limit wasexceeded. Extremely short interruptions do not effect the circuitbecause of the time constant inherent in the circuit which includes thecapacitor 200, and the resistors 202 and 203 through which the capacitor200 is discharged following an interruption in power. If the powerinterruption is so shortthat the voltage level at the gate of the FET204 does not pass the cutoff threshold, the capacitor 206 does notbecome discharged and the FET 212 remains nonconductive. In situationswhere it is desired to detect a low limit or for other applications, thedrain of FET 212 may be coupled to the inverting input 122 of amplifier120 rather than noninverting input 124.

Referring now to FIG. 3, there is shown an alternative embodiment which,in combination with the apparatus of FIG. 1, is effective to produce anoutput signal in proportion to the amount of control desired. Inparticular, the range of temperatures over which the controller isoperative may be'defined as the span of the controller. The controllerset point is within the span and is also located within a second rangeof temperatures defined as the proportional bandwidth. The proportionalbandwidth is normally of a much smaller temperature range than the span.In one apparatus of FIG. 3, the bandwidth is two percent of the span.The controller output signal assumes a first state when the temperaturesensed by thermocouple 12 is less than the temperatures included withinthe proportional bandwidth, and a second state when the sensedtemperature is greater than the temperatures included within theproportional bandwidth. When the sensed temperature is within theproportional bandwidth, the controller output alternates between the twostates, creating a square wave output having a duty cycle which isrelated to the magnitude of the difference between the sensed and setpoint temperatures. If the duty cycle is defined as the percentage oftime the controller output assumes the first state, the duty cycle willdecrease with increasing temperature. The controller may establish alinear relation between temperature within the proportional bandwidthand duty cycle, but nonlinear relationships are often desirable.

The apparatus of FIG. 3 is designed to be connected to the apparatus ofFIG. 1, in place of the apparatus of FIG. 2 which has been describedabove. The input signal is made available at a terminal 112, and passesthrough a resistor 218 to the inverting input 220 of an operationalamplifier 222. A pair of reverse-poled diodes 224 and 226 are connectedbetween the input 220 and ground, for limiting. A capacitor 228 and aresistor 230 are connected in parallel between the inverting input 220and ground. Positive potential is applied to the line 132 and a negativepotential is applied to the line 154 to furnish the power required bythe operational amplifier 222 at terminals 232 and 233 thereof, just asdescribed in connection with FIG. 2. Also as described in connectionwith FIG. 2, capacitors 234 and 235 connect terminals 232 and 233,respectively, to ground.

The output is made available at an output terminal 238 and a voltagedivider circuit including serially connected resistors 240 and 242 isconnected between the terminal 238 and ground. The junction of theresistors 240 and 242 is connected via resistor 244 to the invertinginput 220 of the amplifier 222 to furnish negative feedback. Positivefeedback is supplied via a line 246 connecting the noninverting input248 of the amplifier 222 to the junction of two resistors 250 and 252which are connected in series from the terminal 238 to ground, to form asecond voltage divider. A resistor 254 connects the noninverting input248 to either the line 132 or the line 154 as may be required. Its valueis selected to adjust the duty cycle of the controller output signal toa desired level when a zero level signal is applied to input 112, i.e.,when the sensed temperature equals the set point temperature.

The output of the operational amplifier 222 is connected by means of theoutput resistor 164 to the gate of an SCR 166,just as is the circuit ofFIG. 2. Like FIG. 2, the gate of the SCR 166 is connected to ground by aresistor 168, and is extinguished each half cycle by the a.c. voltagesupplied to it via terminal 170.

The circuit of FIG. 3 forms an astable multivibrator in which a squarewave is made available at the output terminal 238 when the input voltageat the terminal 112 is at ground potential. As mentioned, the duty cycleof that square wave is selected by altering the value of resistor 254.The oscillation of the circuit is caused by the capacitor 228. Duringone portion of each cycle, the capacitor 228 holds the potential of theterminal 220 at a positive value, providing negative output at theterminal 238 and corresponding to a first state of the controlleroutput, but is charged negatively by negative feedback through theresistor 244. When the capacitor 228 attains sufficient negative charge,the state of conduction of the amplifier 222 is reversed, and a positivepotential is produced at the output terminal 238 corresponding to asecond state of the controller output. This in turn charges thecapacitor 228 in a positive direction through the resistor 244 until theconduction state is reversed again. The amount of positive feedbackthrough the resistor 250 determines how much the capacitor 228 must becharged in order to reverse the state of conduction.

When the voltage level at the input terminal 112 rises, the capacitor228 tends to be charged in a positive direction through the resistor 218so that less time is required, during a positive swing of the capacitor228, before the state of conduction of the amplifier 222 is reversed.Similarly, more time is required during a negative swing to reverse theconduction state. As a result, the output signal available at theterminal 238 is asymmetrical and has a negative value for a greater timeduring each cycle than its positive value.

When the voltage level on the terminal 112 goes negative, the duty cycleof the amplifier unit 222 is moditied in the opposite .way to producepositive-going portions of greater duration during each cycle. Thesquare wave thus produced causes the relay 176, connected in the samemanner as shown in FIG. 2, to operate so as to execute its controlfunction in a proportional way, being closed for a greater or lesseramount of time during each cycle, as required. If the actual temperatureas measured by the thermocouple 12 (FIG. 1) differs sufficiently fromthe temperature set by the potentiometer 28, that the sensed temperatureis outside the proportional bandwidth, the signal at terminal 238 willbe continually positive (corresponding to a one hundred percent dutycycle) or continually negative (corresponding to a zero percent dutycycle), depending on whether the temperature is less or greater,respectively, than the set point. Thus, when the temperature is outsidethe proportional bandwidth, the duty cycle of the signal produced at theterminal 238 is shifted to its maximum extent, i.e., to either zero orone hundred percent. When the sensed temperature comes within theproportional bandwidth, the duty cycle is shifted and eventually astable condition is reached when the desired temperature approximatesthe temperature set by the potentiometer 28.

There will, however, normally be some residual error remaining after thecontroller has stabilized. As will be recalled, the circuit constantsassociated with the apparatus of FIG. 3 were selected to give a desiredduty cycle when the sensed temperature is equal to the set pointtemperature. However, in most instances, that selected duty cycle willnot result in achievement of the set point temperature, but will resultin some other temperature within the proportional bandwidth. Thus, thecontroller may stabilize at some temperature other than the set pointtemperature yielding a signal at terminal 112 other than zero and a dutycycle other than that desired. However, the temperature deviation fromthe set point is normally small, and may be made arbitrarily small byreducing the range of the proportional bandwidth, so that in manyapplications it is not a serious disadvantage.

. Resistor 244 provides an approximately linear relationship between thedeviation of the sensed temperature from the set point temperature andthe duty cycle within the proportional bandwidth. An alternativefeedback circuit is shown in FIG. 3. It includes a series diode 260 andresistor 262, and a series diode 264 and a resistor 266, connected inlieu of the resistor 244. The diodes 260 and 264 are reverse poled sothat the rate of feedback current during alternate half cycles may beindependently controlled by selecting appropriate values for theresistors 262 and 266. The independent feedback rates for alternate halfcycles permit adjustment of the relative position of the set pointwithin the proportional bandwidth. Further, the diodes may be used tocreate a nonlinear relationship between sensed temperature deviation andduty cycle.

In the circuit of FIG. 3, the proportional bandwidth of the controllermay be adjusted by altering the value of resistor 218 and thus the rateof current flow between capacitor 228 and terminal 112. Decreasing theresistance of resistor 218 increases that current flow, reducing therelative effect of the feedback through resistor 244, thus decreasingthe range of the proportional bandwidth. Conversely, increasing thevalue of resistor 218 will increase the proportional bandwidth. Therelative cycle times, as exemplified by the period of the controlleroutput at fifty percent duty cycle, may be adjusted by altering thevalues of resistors 240, 242, 244 or 250 or capacitor 228. Placing allof the relative timing adjustments of the controller in conjunction withone operational amplifier 222 aids in the stabilization of thecontroller operation with changes in such parameters as power supplyvoltages and ambient temperature.

In the foregoing, the control circuits of the present invention havebeen described particularly in relation to the control of temperature,and especially with a thermocouple as the temperature sensing device.However, it will be appreciated by those skilled in the art that thecontrol circuitry of the present invention can also be employed toexecute a control function in response to parameters other thantemperature, and that another sensing device may be employed instead ofa thermocouple.

While specific embodiments of the invention have been described, it willbe obvious that certain modifications can be made without departing fromthe spirit of the invention, and it is intended to include suchmodifications within the scope of the invention.

What is claimed is:

1. A controller comprising a source of electrical power, thermocouplemeans, operational amplifier means connected to said source and to saidthermocouple means and responsive to said thermocouple means formanifesting a first output signal when the temperature of saidthermocouple means is below a predetermined limit, and a second outputsignal when the temperature of said thermocouple means is above saidpredetermined limit, sensing means connected to said source of power forsensing a restoration of power after an interruption thereof, and meansfor connecting said sensing means with said operational amplifier forforcing said amplifier to manifest said first output signal for apredetermined interval following said restoration.

2. A controller according to claim 1 wherein said sensing meanscomprises delay means having a time constant corresponding to saidpredetermined interval, and selectively conductive means connected tosaid delay means and responsive thereto to assume its conductive stateduring said predetermined interval.

3. A controller according to claim 1 wherein said sensing meanscomprises a first transistor connected to said power source, connectingmeans for connecting said first transistor to said operational amplifierto force said amplifier to assume its first condition when said firsttransistor is conductive, a first timing circuit connected with saidpower supply and with a control element of said first transistor, asecond transistor connected with said power supply and with said firsttiming circuit for selectively disabling said first timing circuit whensaid second transistor becomes conductive, and a second timing circuitinterconnected with said power supply and a control element of saidsecond transistor to cause said second transistor to becomenonconductive at a predetermined time following said restoration, saidfirst timing circuit being responsive to the operation of said secondtransistor to cause said first transistor to become nonconductive at apredetermined time following the time when said second transistorbecomes nonconductive.

4. A controller according to claim 3 wherein said second timing circuitcomprises a resistor connected with said power supply, and a capacitorconnected to said resistor and adapted to be charged by. said powersupply through said resistor, and means for connecting said capacitorand said resistor to the gate of said second transistor for disablingsaid second transistor at said predetermined time.

5. A controller comprising sensing means for producing an electricalsignal in response to the value of a physical parameter, an outputterminal, and means connected to said sensing means and responsive tosaid electrical signal for producing a proportional signal at saidoutput terminal, said proportional signal having a duty cycle related tothe deviation of the sensed level of said parameter from a preselectedlevel over a given range of said parameter, said last named meanscomprising an operational amplifier, a first voltage divider connectedto the output of said operational amplifier, means for connecting saidfirst voltage divider to the inverting input of said amplifier forsupplying negative feedback to said amplifier, a second voltage dividerconnected with the output of said amplifier, means for connecting saidsecond voltage divider to the noninverting input of said amplifier forsupplying positive feedback to said amplifier, and capacitor meansconnected between said inverting input and a source of referencepotential.

6. A controller according to claim wherein said negative feedback meanscomprises a first resistor connected in series with a first diodebetween said first voltage divider and said inverting input, and asecond resistor connected in series with a second diode between saidfirst voltage divider and said inverting input, said first and seconddiodes being connected in oppositely poled relationship.

7. A controller according to claim 5 including a source of a.c. power, asilicon controlled rectifier having its anode and cathode connected inseries with said source, and means for connecting said output terminalto the gate terminal of said silicon controlled rectifier whereby ac.current through said silicon controlled rectifier is controlled inresponse to said proportional signal.

8. A controller according to claim 5 wherein said sensing means includesthermocouple means for producing an electrical signal in response to thetemperature of the junction of said thermocouple means, and

generating means connected to said thermocouple means and responsive tosaid electrical signal for producing an output signal,

said generating means including a source of regulated voltage,

a first adjustable voltage divider connected between said regulatedvoltage and a source of reference potential,

connecting means for connecting said first adjustable voltage divider toa first input of a differential am- 14 plifier,

a second adjustable voltage divider connected between said source ofregulated voltage and said reference potential,

one terminal of said thermocouple means being connected with said secondadjustable voltage divider and the other terminal of said thermocouplemeans being connected to the other input of said differential amplifier,whereby the differential in the potential of the two inputs to saiddifferential amplifier is proportional to the temperature of saidjunction.

9. A controller according to claim 8 including a negative temperaturecoefficient resistor element connected in series with said secondadjustable voltage divider, said resistor elementt-being positionedadjacent the connection of said thermocouple means with said secondvoltage divider and the connection of said thermocouple means with saiddifferential amplifier.

10. A controller according to claim 8 including a rheostat, saidrheostat being connected across a portion of said second adjustablevoltage divider, said rheostat establishing a range of potentials whichmay be selected by adjustment of said second voltage divider.

11. A controller comprising a source of electrical power, parametersensing means, operational amplifier means connected to said source andto said parameter sensing means and responsive to said parameter sensingmeans for manifesting a first output signal when the parameter sensed bysaid parameter sensing means is below a predetermined limit, and asecond output signal when the parameter sensed by said parameter sensingmeans is above said predetermined limit, power sensing means connectedto said power source of power for sensing a restoration of power afteran interruption thereof, and means for connecting said power sensingmeans with said operational amplifier for forcing said amplifier tomanifest said first output signal for a predetermined interval followingsaid restoration.

1. A controller comprising a source of electrical power, thermocouplemeans, operational amplifier means connected to said source and to saidthermocouple means and responsive to said thermocouple means formanifesting a first output signal when the temperature of saidthermocouple means is below a predetermined limit, and a second outputsignal when the temperature of said thermocouple means is above saidpredetermined limit, sensing means connected to said source of power forsensing a restoration of power after an interruption thereof, and meansfor connecting said sensing means with said operational amplifier forforcing said amplifier to manifest said first output signal for apredetermined interval following said restoration.
 2. A controlleraccording to claim 1 wherein said sensing means comprises delay meanshaving a time constant corresponding to said predetermined interval, andselectively conductive means connected to said delay means andresponsive thereto to assume its conductive state during saidpredetermined interval.
 3. A controller according to claim 1 whereinsaid sensing means comprises a first transistor connected to said powersource, connecting means for connecting said first transistor to saidoperational amplifier to force said amplifier to assume its firstcondition when said first transistor is conductive, a first timingcircuit connected with said power supply and with a control element ofsaid first transistor, a second transistor connected with said powersupply and with said first timing circuit for selectively disabling saidfirst timing circuit when said second transistor becomes conductive, anda second timing circuit interconnected with said power supply and acontrol element of said second transistor to cause said secondtransistor to become nonconductive at a predetermined time followingsaid restoration, said first timing circuit being responsive to theoperation of said second transistor to cause said first transistor tobecome nonconductive at a predetermined time following the time whensaid second transistor becomes nonconductive.
 4. A controller accordingto claim 3 wherein said second timing circuit comprises a resistorconnected with said power supply, and a capacitor connected to saidresistor and adapted to be charged by said power supply through saidresistor, and means for connecting said capacitor and said resistor tothe gate of said second transistor for disabling said second transistorat said predetermined time.
 5. A controller comprising sensing means forproducing an electrical signal in response to the value of a physicalparameter, an output terminal, and means connected to said sensing meansand responsive to said electrical signal for producing a proportionalsignal at said output terminal, said proportional signal having a dutycycle related to the deviation of the sensed level of said parameterfrom a preselected level over a given range of said parameter, said lastnamed means comprising an operational amplifier, a first voltage dividerconnected to the output of said operational amplifier, means forconnecting said first voltage divider to the inverting input of saidamplifier for supplying negative feedback to said amplifier, a secondvoltage divider connected with the output of said amplifier, means forconnecting said second voltage divider to the noninverting input of saidamplifier for supplying positive feedback to said amplifier, andcapacitor means connected between said inverting input and a source ofreference potential.
 6. A controller according to claim 5 wherein saidnegative feedback means comprises a first resistor connected in serieswith a first diode between said first voltage divider and said invertinginput, and a second resistor connected in series with a second diodebetween said first voLtage divider and said inverting input, said firstand second diodes being connected in oppositely poled relationship.
 7. Acontroller according to claim 5 including a source of a.c. power, asilicon controlled rectifier having its anode and cathode connected inseries with said source, and means for connecting said output terminalto the gate terminal of said silicon controlled rectifier whereby a.c.current through said silicon controlled rectifier is controlled inresponse to said proportional signal.
 8. A controller according to claim5 wherein said sensing means includes thermocouple means for producingan electrical signal in response to the temperature of the junction ofsaid thermocouple means, and generating means connected to saidthermocouple means and responsive to said electrical signal forproducing an output signal, said generating means including a source ofregulated voltage, a first adjustable voltage divider connected betweensaid regulated voltage and a source of reference potential, connectingmeans for connecting said first adjustable voltage divider to a firstinput of a differential amplifier, a second adjustable voltage dividerconnected between said source of regulated voltage and said referencepotential, one terminal of said thermocouple means being connected withsaid second adjustable voltage divider and the other terminal of saidthermocouple means being connected to the other input of saiddifferential amplifier, whereby the differential in the potential of thetwo inputs to said differential amplifier is proportional to thetemperature of said junction.
 9. A controller according to claim 8including a negative temperature coefficient resistor element connectedin series with said second adjustable voltage divider, said resistorelement being positioned adjacent the connection of said thermocouplemeans with said second voltage divider and the connection of saidthermocouple means with said differential amplifier.
 10. A controlleraccording to claim 8 including a rheostat, said rheostat being connectedacross a portion of said second adjustable voltage divider, saidrheostat establishing a range of potentials which may be selected byadjustment of said second voltage divider.
 11. A controller comprising asource of electrical power, parameter sensing means, operationalamplifier means connected to said source and to said parameter sensingmeans and responsive to said parameter sensing means for manifesting afirst output signal when the parameter sensed by said parameter sensingmeans is below a predetermined limit, and a second output signal whenthe parameter sensed by said parameter sensing means is above saidpredetermined limit, power sensing means connected to said power sourceof power for sensing a restoration of power after an interruptionthereof, and means for connecting said power sensing means with saidoperational amplifier for forcing said amplifier to manifest said firstoutput signal for a predetermined interval following said restoration.